The present invention pertains to a method for controlling a blow-molding machine for the production of a container of thermoplastic material, to a control device for controlling a blow-molding machine for the production of containers of thermoplastic material, and to a blow-molding machine for the production of containers of thermoplastic material.
In methods of the type indicated above, a blowing gas or blowing air is introduced into a thermally preconditioned preform of thermoplastic material to blow-mold a container. The blow-molding process typically proceeds in two stages. In the first or “preblowing” phase, the preform is subjected to a first pressure of, for example, 10 bars, so that the temperature-conditioned material of the preform expands uniformly in a controlled manner and can come to rest against the walls of the blow mold. In the second or “finish-blowing” phase, the preexpanded preform is subjected to a second pressure of, for example, 40 bars, so that the material can conform completely to the contours of the blow mold being used.
Because the pressure of the process gases at a blow-molding machine are normally in the range of 40 bars, the blowing pressure in conventional systems is reduced during the preblowing phase to approximately 10 bars by a pressure control valve.
During the preblowing phase, the blowing gas is introduced into the preform through a flow-control valve, for example, with an open cross section of, for example, 4 mm2. The setting of the flow-control valve depends on various factors such as the type of material of the container to be produced, the temperature profile with which the preform was thermally conditioned, the wall thickness of the preform, and other parameters as well. The degree to which the flow-control valve is opened is typically adjusted manually a single time and remains unchanged after that. Another manual change of the valve cross section can be necessary, however, when a flow-control valve has become dirty over the course of its service life, for example, or the blow-molding machine is to be set up for a different blow mold or a different preform.
The disadvantage with this prior art is that, first, the pressure of the blowing air for the preblowing phase must be adjusted downward, and, second, the volumetric flow rate of the blowing air is not adjustable during either the preblowing or the finish-blowing phase.